Light emitting device

ABSTRACT

A triplet light emitting device which has high efficiency and improved stability and which can be fabricated by a simpler process is provided by simplifying the device structure and avoiding use of an unstable material. In a multilayer device structure using no hole blocking layer conventionally used in a triplet light emitting device, that is, a device structure in which on a substrate, there are formed an anode, a hole transporting layer constituted by a hole transporting material, an electron transporting and light emitting layer constituted by an electron transporting material and a dopant capable of triplet light emission, and a cathode, which are laminated in the stated order, the combination of the hole transporting material and the electron transporting material and the combination of the electron transporting material and the dopant material are optimized.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an organic light emitting deviceconstituted by an anode, an organic compound film capable of emittinglight under the action of an electric field, and a cathode. Inparticular, the present invention relates to an organic light emittingdevice using a light emitting material which emits light in a tripletexited state.

[0003] 2. Description of the Related Art

[0004] An organic light emitting device is a device designed byutilizing a phenomenon in which electrons and holes are caused to flowinto an organic compound film through two electrodes by application of avoltage to cause emission of light from molecules in an excited state(excited molecules) formed by recombination of the electrons and holes.

[0005] Emission of light from an organic compound is a conversion intolight of energy released when excited molecules are formed and thendeactivated into the ground state. Deactivation processes causing suchemission of light are broadly divided into two kinds: a process in whichdeactivation proceeds via a state in which excited molecules are singletexcited molecules (in which fluorescence is caused), and a process inwhich excited molecules are triplet excited molecules. Deactivationprocesses via the triplet excited molecule state include an emissionprocess in which phosphorescence is caused and a triplet-tripletextinction process. However, there are basically only a small number oforganic materials capable of changing in accordance with thephosphorescent deactivation process at room temperature. (In ordinarycases, thermal deactivation different from deactivation with emission oflight occurs.) The majority of organic compounds used in organic lightemitting devices are materials which emit light by fluorescence via thesinglet excited molecule state, and many organic light emitting devicesuse fluorescence.

[0006] Organic light emitting devices using such organic compoundscapable of emitting light by fluorescence are based on the two-layerstructure which was reported by C. W. Tang et al. in 1987 (Reference 1:C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”,Applied Physics Letters, Vol. 51, No.12, 913-915 (1987)), and in whichan organic compound film formed of layers of two or more organiccompounds and having a total thickness of about 100 nm is interposedbetween electrodes. Adachi et al. thereafter proposed a three-layerstructure in 1988 (Reference 2: Chihaya ADACHI, Shozuo TOKITO, TetsuoTSUTSUI and Shogo SAITO, “Electroluminescence in Organic Films withThree-Layered Structure”, Japanese Journal of Applied Physics, Vol. 27,No.2, L269-L271 (1988)). Multilayer device structures based onapplications of these layered structures are being presently used.

[0007] Devices in such multilayer structures are characterized by “layerfunction separation”, which refers to the method of separately assigningfunctions to layers, instead of making one organic compound have variousfunctions. For example, a device of two-layer structure uses a holetransporting layer having the function of transporting positive holes,and a light emitting and electron transporting layer having the functionof transporting electrons and the function of emitting light. Also, adevice of three-layer structure uses a hole transporting layer havingonly the function of transporting positive holes, an electrontransporting layer having only the function of transporting electrons,and a light-emitting layer which is capable of emitting light, and whichis formed between the two transporting layers. Such a layer functionseparation method has the advantage of increasing the degree ofmolecular design freedom of organic compounds used in an organic lightemitting device.

[0008] For example, a number of characteristics, such as improvedfacility with which either of electrons and holes are injected, thefunction of transporting both the carriers, and high fluorescent quantumyield, are required of a device of single-layer structure. In contrast,in the case of a device of two-layer structure or the like using anelectron transporting and light emitting layer, an organic compound towhich positive holes can be easily injected may be used as a materialfor a hole transporting layer, and an organic compound to whichelectrons can be easily injected and which have high fluorescent quantumyield may be used as a material for an electron transporting layer.Thus, requirements of one layer are reduced and the facility with whichthe material is selected is improved.

[0009] In the case of a device of three-layer structure, a “lightemitting layer” is further provided to enable separation between theelectron transporting function and the light emitting function.Moreover, a material in which a fluorescent pigment (guest) of highquantum yield such as a laser pigment is dispersed in a solid medium(host) material can be used for the light emitting layer to improve thefluorescent quantum yield of the light emitting layer. Thus, not onlythe effect of largely improving the quantum yield of the organic lightemitting device but also the effect of freely controlling the emissionwavelength through the selection of fluorescent pigments to be used canbe obtained (Reference 3: C. W. Tang, S. A. Vanslyke and C. H. Chen,“Electroluminescence of doped organic thin films”, Journal of AppliedPhysics, Vol. 65, 3610-3616 (1989)). A device in which such a pigment(guest) is dispersed in a host material is called a doped device.

[0010] Another advantage of the multilayer structure is a “carrierconfinement effect”. For example, in the case of the two-layer structuredescribed in Reference 1, positive holes are injected from the anodeinto the hole transporting layer, electrons are injected from thecathode into the electron transporting layer, and the holes andelectrons move toward the interface between the hole transporting layerand the electron transporting layer. Thereafter, while holes areinjected into the electron transporting layer because of a smallionization potential difference between the hole transporting layer andthe electron transporting layer, electrons are blocked by the holetransporting layer to be confined in the electron transporting layerwithout being injected into the hole transporting layer because theelectrical affinity of the hole transporting layer is low and becausethe difference between the electrical affinities of the holetransporting layer and the electron transporting layer is excessivelylarge. Consequently, both the density of holes and the density ofelectrons in the electron transporting layer are increased to achieveefficient carrier recombination.

[0011] As an example of a material that is effective in exhibiting thecarrier confinement effect, there is a material having an extremelylarge ionization potential. It is difficult to inject holes into thematerial having a large ionization potential, so that such a material iswidely used as a material capable of blocking holes (hole blockingmaterial). For example, in the case where the hole transporting layercomposed of an aromatic diamine compound and the electron transportinglayer composed of tris(8-quinolinolato)-aluminum (hereinafter referredto as “Alq”) are laminated as reported in Reference 1, if a voltage isapplied to the device, Alq in the electron transporting layer emitslight. However, by inserting the hole blocking material between the twolayers of the device, holes are confined in the hole transporting layer,so that light can be emitted from the hole transporting layer side aswell.

[0012] As described above, layers having various functions (holetransporting layer, hole blocking layer, electron transporting layer,electron injection layer, etc.) are provided to improve the efficiencyand to enable control of the color of emitted light, etc. Thus,multilayer structures have been established as the basic structure forcurrent organic light emitting devices.

[0013] Under the above-described circumstances, in 1998, S. R. Forrestet al. made public a doped device in which a triplet light emittingmaterial capable of emission of light (phosphorescence) from a tripletexcited state at a room temperature (a metal complex having platinum asa central metal in the example described in the reference) is used as aguest (hereinafter referred to as “triplet light emitting device)(Reference 4: M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustilkov, S.Silbley, M. A. Thomoson and S. R. Forrest, “Highly efficientphosphorescent emission from organic electroluminescent devices”,Nature, Vol. 395, 151-154 (1998)). For distinction between this tripletlight emitting device and devices using emission of light from a singletexcited state, the latter device will be referred to as “singlet lightemitting device”.

[0014] As mentioned above, excited molecules produced by recombinationof holes and electrons injected into an organic compound include singletexcited molecules and triplet excited molecules. In such a case, singletexcited molecules and triplet excited molecules are produced inproportions of 1:3 due to the difference between the multiplicities ofspin. Basically, in the existing materials, triplet excited moleculesare thermally deactivated at room temperature. Therefore only singletexcited molecules have been used for emission of light, only a quarterof produced excited molecules are used for emission of light. If tripletexcited molecules can be used for emission of light, a light emissionefficiency three to four times higher than that presently achieved canbe obtained. In Reference 4, the above-described multilayer structure isused. That is, the device is such structured that:4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (hereinafter referredto as “α-NPD”) that is an aromatic amine-based compound, is used as thehole transporting layer; Alq with 6% of2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin-platinum (hereinafterreferred to as “PtOEP”) dispersed therein is used as the light emittinglayer; and Alq is used as the electron transporting layer. As to theexternal quantum efficiency, the maximum value is 4% and a value of 1.3%is obtained at 100 cd/m².

[0015] Thereafter, the device structure utilizing the hole blockinglayer is used. That is, the device is such structured that: α-NPD isused as the hole transporting layer; 4,4′-N,N′-dicarbazole-biphenyl(hereinafter referred to as “CBP”) with 6% of PtOEP dispersed therein isused as the light emitting layer;2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred toas “BCP”) is used as the hole blocking layer; and Alq is used as theelectron transporting layer. As to the external quantum efficiency, avalue of 2.2% is obtained at 100 cd/m² and the maximum value is 5.6%, sothat the light emission efficiency of the device is improved (Reference5: D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest,“Improved energy transfer in electrophosphorescent devices”, AppliedPhysics Letters, Vol. 74, No. 3, 442-444 (1999)).

[0016] Further, a triplet light emitting device is reported which usestris(2-phenylpyridine)iridium (hereinafter referred to as “Ir(ppy)₃”) asthe triplet light emitting material (Reference 6: M. A. Baldo, S.Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, “Veryhigh-efficiency green organic light-emitting devices based onelectrophosphorescence”, Applied Physics Letters, Vol. 75, No. 1, 4-6(1999)). Thereafter, it is reported that with the same device structureas in Reference 6, the film thicknesses of the organic compound filmsare optimized, whereby a highly efficient organic light emitting deviceis obtained whose external quantum efficiency is 14.9% at 100 cd/m²(Reference 7: Teruichi Watanabe, Kenji Nakamura, Shin Kawami, YoshinoriFukuda, Taishi Tsuji, Takeo Wakimoto, Satoshi Miyaguchi, MasayukiYahiro, Moon-Jae Yang, Tetsuo Tsutsui, “Optimization of emittingefficiency in organic LED cells using Ir complex”, Synthetic Metals,Vol. 122, 203-207 (2001)). Thus, in actuality, it becomes possible toproduce the devices with the light emission efficiency almost threetimes that in the conventional singlet light emitting device.

[0017] Searches are presently being made for triplet light emittingmaterials using iridium or platinum as a central metal, triplet lightemitting devices having markedly high efficiency in comparison withsinglet light emitting devices are now attracting attention, and studiesabout such devices are being energetically made.

[0018] Although triplet light emitting devices have light emissionefficiency much higher than that of singlet light emitting devices, theyare incomparably shorter in life than singlet light emitting materialsand lack stability. Also, a multilayer structure adopted to increase theefficiency of a triplet light emitting device must be formed so as tohave at least four layers. Therefore triplet light emitting devicessimply have the drawback of requiring much time and labor forfabrication.

[0019] With respect to the life of devices, a report has been made whichsays that the half-life of a device having a multilayer structure formedof a hole transporting layer using α-NPD, a light emitting layer usingCBP as a host material and Ir(ppy)₃ as a guest (dopant) material, a holeblocking layer using BCP, and an electron transporting layer using Alqis only 170 hours under a condition of an initial luminance of 500 cd/m²(Reference 8: Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, KenjiNAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, TakeoWAKIMOTO and Satoshi MIYAGUCHI, “High Quantum Efficiency InorganicLight-Emitting Devices with Iridium-Complex as a Triplet EmissiveCenter”, Japanese Journal of Applied Physics, Vol. 38, No.12B,L1502-L1504 (1999)). By considering this life, it must be said that nosolution of the life problem is close at hand.

[0020] In Reference 8, low stability of BCP used as a hole blockingmaterial is mentioned as a cause of the limitation of life. Tripletlight emitting devices use as a basic structure the device structuredescribed in Reference 5, and use the hole blocking layer as anindispensable element. FIG. 12 show the structure of a conventionaltriplet light emitting device. In the device structure shown in FIG. 12,an anode 1102 is formed on a substrate 1101, a multilayer organiccompound film formed of a hole transporting layer 1103, a light emittinglayer 1104, a hole blocking layer 1105, and an electron transportinglayer 1106 is formed on the anode 1102, and a cathode 1107 is formed onthe multilayer film. While efficient carrier recombination can beachieved by the carrier confinement effect of the hole blocking layer,the life of the device is limited because the hole blocking materialordinarily used is considerably low in stability. Also, CBP used as ahost material is also low in stability and is also considered to be acause of the limitation of the life.

[0021] A device of three-layer structure using no hole blocking layerhas been fabricated (Reference 9: Chihaya ADACHI, Marc A. Baldo, StephenR. Forrest and Mark E. Thompson, “High-efficiency organicelectrophosphorescent devices with tris(2-phinylpyridine) iridium dopedinto electron-transporting materials”, Applied Physics Letters, Vol. 77,No.6, 904-906 (2000)). This device is characterized by using electrontransporting materials as a host material instead of CBP which is the tohave such characteristics as to transport both the carriers. However,the electron transporting materials used as a host material are BCPwhich is used as a hole blocking material,1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxazole (hereinafter referred to as“OXD7”), and 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole(hereinafter referred to as “TAZ”). Although no hole blocking layer isused, the materials ordinarily used as a hole blocking material are usedin the device. BCP is, of course, lower in stability than any othermaterial, so that the stability of the device is low, while theefficiency is high.

[0022] A simple two-layer device structure using no hole blockingmaterial has also been reported (Reference 10: Chihaya ADACHI, RaymondKWONG, Stephen R. Forrest, “Efficient electrophosphorescence using adoped ambipolar conductive molecular organic thin film”, OrganicElectronics, Vol. 2, 37-43 (2001)). In this device, however, CBP is usedas a host material, so that the stability is low, while the lightemission efficiency is high.

[0023] As described above, while triplet light emitting devices havinghigh light emission efficiency have been reported, no triplet lightemitting device improved both in efficiency and in stability has beenreported. Difficulty in obtaining such an improved device is due to theinstability of host materials and hole blocking materials used.

SUMMARY OF THE INVENTION

[0024] An object of the present invention is to provide a triplet lightemitting device in which unstable materials such as those describedabove are not used while the device structure is simplified to obtainhigh efficiency and improved stability, and which can be fabricatedeasily and efficiently in comparison with the conventional devices.

[0025] According to the present invention, a triplet light emittingdevice designed to achieve the above-described object has a simpledevice structure (FIG. 1) in which no hole blocking layer such as thatprovided in the conventional triplet light emitting devices is used, andin which an organic compound film is formed as a multilayer filmconstituted by a hole transporting layer and a layer in which dopantmaterial capable of triplet light emission is dispersed in a stableelectron transporting material. That is, a device structure is providedin which an anode 102 is formed on a substrate 101, a hole transportinglayer 103 constituted by a hole transporting material, an electrontransporting and light emitting layer 104 constituted by an electrontransporting material and a dopant material capable of triplet lightemission are successively formed on the anode 102, and a cathode 105 isformed on the layer 104. The region interposed between the anode 102 andthe cathode 105 (i.e., the hole transporting layer 103 and the electrontransporting and light emitting layer 104) corresponds to the organiccompound film.

[0026] The present invention is characterized in that, in an organiclight emitting device constituted by an anode, an organic compound film,and a cathode, the organic compound film includes a hole transportinglayer constituted by a hole transporting material, and an electrontransporting layer formed adjacent to the hole transporting layer andconstituted by an electron transporting material, and a light emittingmaterial capable of emitting light from a triplet excited state is addedin the electron transporting layer.

[0027] A hole injection layer may be inserted between the anode 102 andthe hole transporting layer 103. Also, an electron injection layer maybe inserted between the cathode 105 and the electron transporting andlight emitting layer 104. Further, both the hole injection layer and theelectron injection layer may be inserted.

[0028] As a means for achieving the object of the present invention, itis important to consider the combination of a hole transporting materialand an electron transporting material in the above-described device forpreventing emission of light from the hole transporting layer 103.

[0029] Accordingly, the present invention is characterized in that theenergy difference between the highest occupied molecular orbit level andthe lowest unoccupied molecular orbit level in the hole transportingmaterial is larger than the energy difference between the highestoccupied molecular orbit level and the lowest unoccupied molecular orbitlevel in the electron transporting material.

[0030] Another means for achieving the object resides in avoidingoverlap between an absorption spectrum of the hole transporting materialand a light emission spectrum of the electron transporting material. Inthis case, it is preferred not only that the spectrums do not overlapeach other, but also that the positional relationship between thespectrums be such that the absorption spectrum of the hole transportingmaterial is on the shorter-wavelength side of the light emissionspectrum of the electron transporting material.

[0031] As a means for achieving the object of the present invention, itis important to adopt a device arrangement enabling the dopant capableof triplet light emission to easily trap the carriers in improving thelight emission efficiency of the above-described triplet light emittingdevice of the present invention.

[0032] Accordingly, the present invention is characterized in that boththe highest occupied molecular orbit level and the lowest unoccupiedmolecular orbit level of the light emitting material capable of emittinglight from a triplet excited state are in the energy gap between thehighest occupied molecular orbit level and the lowest unoccupiedmolecular orbit level of the electron transporting material.

[0033] As still another means for achieving the object of the presentinvention, the light emitting device is characterized in that the valueof ionization potential of the hole transporting material is equal to orlarger than the value of ionization potential of the light emittingmaterial capable of emitting light from a triplet excited state.

[0034] Further, as another means for achieving the object of the presentinvention, the light emitting device is characterized in that theabsolute value of a value indicating the lowest unoccupied molecularorbit level of the hole transporting material is smaller by 0.2 eV ormore than the absolute value of a value indicating the lowest unoccupiedmolecular orbit level of the electron transporting material.

[0035] It is more preferable to use a device arrangement correspondingto a combination of these means, i.e., an arrangement in which the valueof ionization potential of the hole transporting material is equal to orlarger than the value of ionization potential of the light emittingmaterial capable of emitting light from a triplet excited state, and theabsolute value of a value indicating the lowest unoccupied molecularorbit level of the hole transporting material is smaller by 0.2 eV ormore than the absolute value of a value indicating the lowest unoccupiedmolecular orbit level of the electron transporting material.

[0036] In view of the above description, the present invention ischaracterized in that used as the preferred hole transporting materialis one selected from the group consisting of4,4′,4″-tris(N-carbazole)triphenylamine,4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane, 1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene,1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene, and1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene.

[0037] Further, the present invention is characterized in that used asthe electron transporting material is one selected from the groupconsisting of2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzoimidazole],lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron,bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum,bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum,2-(2-hydroxyphenyl)benzooxazolatolithium,(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron,tris(8-quinolinolato)-aluminum,bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum,lithiumtetra(2-methyl-8-hydroxy-quinolinato)boron,(2-methyl-8-quinolinolato)-diphenylboron, andbis(2-methyl-8-quinolinolato)aluminiumhydroxide.

[0038] Further, in the device of the present invention, it is effectiveto use the hole transporting material and the electron transportingmaterial in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In the accompanying drawings:

[0040]FIG. 1 is a diagram showing the device structure of a two-layertriplet light emitting device in accordance with the present invention;

[0041]FIG. 2 is a diagram showing HOMO and LUMO energy levels;

[0042]FIG. 3 is an energy gap diagram of the device;

[0043]FIGS. 4A and 4B are diagrams showing the positional relationshipbetween the light emission spectrum of a host material and theabsorption spectrum of a hole transporting material;

[0044]FIGS. 5A to 5D are graphs showing an initial characteristic and alight emission spectrum in Embodiment 1;

[0045]FIGS. 6A to 6D are graphs showing an initial characteristic and alight emission spectrum in Embodiment 2;

[0046]FIGS. 7A to 7D are graphs showing an initial characteristic and alight emission spectrum in Embodiment 3;

[0047]FIGS. 8A to 8D are graphs showing an initial characteristic and alight emission spectrum in Embodiment 4;

[0048]FIGS. 9A to 9D are graphs showing an initial characteristic and alight emission spectrum in Comparative Example 1;

[0049]FIGS. 10A to 10D are graphs showing an initial characteristic anda light emission spectrum in Comparative Example 2;

[0050]FIGS. 11A to 11D are graphs showing an initial characteristic anda light emission spectrum in Comparative Example 3; and

[0051]FIG. 12 is a diagram showing the device structure of aconventional triplet light emitting device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0052] An embodiment mode of the present invention will be described indetail. An organic light emitting device may have at least one of ananode and a cathode made transparent to enable emitted light to beoutput. However, the embodiment mode of the present invention will bedescribed with respect to a device structure in which a transparentanode is formed on a substrate and light is output through the anode. Inactuality, the present invention can also be applied to a structure inwhich a cathode is formed on a substrate and light is output through thecathode, a structure in which light is output from the side oppositefrom a substrate, and a structure in which light is output throughopposed electrodes.

[0053] As described above, the present invention is characterized inthat use of a hole blocking layer in a triplet light emitting device isavoided (FIG. 1). However, the present invention is different from amethod of fabricating a device designed only by removing the holeblocking layer from the conventional device structure (FIG. 12).

[0054] The conventional triplet light emitting device and the two-layerdevice of the present invention have different recombination regions. Inthe conventional triplet light emitting device, a hole blocking layer isused and the carrier recombination region corresponds to the interfacebetween the light emitting layer and the hole blocking layer. Incontrast, in the device structure proposed in the present invention, thecarrier recombination region corresponds to the interface between thehole transporting layer and the electron transporting material providedas a host material.

[0055] Therefore it is important to consider a light emission mechanismin triplet light emitting devices. In general, two kinds of lightemission mechanisms are conceivable as light emission mechanisms indevices using a guest-host light emitting layer using a dopant (guest).

[0056] The first light emission mechanism is emission from the dopantcaused by transfer of energy from the host. In this case, both thecarriers are injected into the host to form excited molecules of thehost. The energy of the excited molecules is transferred to the dopant.The dopant is excited by the energy and emits light when deactivated. Intriplet light emitting devices, the dopant is a material for emittingphosphorescence via a triplet excited molecule state, and light istherefore emitted by phosphorescence.

[0057] In the light emission mechanism based on transfer of energy, themagnitude of overlap of the light emission spectrum of the host materialand the absorption spectrum of the dopant material is important. Thepositional relationship between the highest occupied molecular orbit(HOMO) and the lowest unoccupied molecular orbit (LUMO) in the hostmaterial and the dopant material is not important.

[0058] In this specification, the value of ionization potential measuredby photoelectron spectrometry in atmospheric air is used as the value ofthe HOMO. The absorption ends of the absorption spectrum define theenergy difference between the HOMO and the LUMO (hereinafter referred toas “energy gap value”). Therefore the value obtained by subtracting theenergy gap value estimated from the absorption ends of the absorptionspectrum from the value of ionization potential measured byphotoelectron spectrometry is used as the value of the LUMO. Inactuality, these values (HOMO (ionization potential), LUMO (energy gapvalue)) are negative since they are measured with reference to thevacuum level. However, they are shown as absolute values throughout thespecification. Conceptual views of the HOMO, the LUMO, and the energygap values are as shown in FIG. 2.

[0059] If both the energy levels of the HOMO and LUMO of the dopantmaterial are placed in the energy gap between the HOMO and LUMO in thehost material, a direct-recombination light emission mechanism, i.e.,direction recombination of the carriers on the dopant when the carriersare trapped on the dopant, occurs as well as the above-described lightemission mechanism based on transfer of energy from the host to thedopant. This is the second light emission mechanism.

[0060] However, in a case where the dopant material and the hostmaterial are in such an energy level relationship, it is ordinarilydifficult to separately determine the contribution of each lightemission mechanism to emission of light since transfer of energy isallowed according to the conditions, and there is a possibility of boththe light emission mechanisms contributing to light emission.

[0061] A case where a triplet light emitting device is emitting light bythe energy transfer mechanism (first light emission mechanism) will bediscussed. In the conventional device structure, since the carrierrecombination region is the interface between the light emitting layerand the hole blocking layer, there is a possibility of transfer ofenergy to the hole blocking material as well as transfer of energy fromthe host material to the dopant material. However, since the absorptionspectrum of the hole blocking material is on an extremely shortwavelength side, there is, therefore, no overlap between the absorptionspectrum of the hole blocking layer and the light emission spectrum ofthe host material reported with respect to the conventional tripletlight emitting devices, and there is no possibility of transfer ofenergy between the host material and the hole blocking material. Thatis, the conventional triplet light emitting devices have such devicestructure that transfer of energy from the host material to the holeblocking material does not occur.

[0062] In contrast, in the device structure in accordance with thepresent invention, the carrier recombination region is the interfacebetween the hole transporting layer containing a hole transportingmaterial and the electron transporting and light emitting layercontaining a host material. In the device of the present invention,therefore, there is a possibility of transfer of energy from the hostmaterial to the hole transporting material. If energy transfer from thehost material to the hole transfer material occurs, efficient emissionof light cannot be achieved.

[0063] The relationship between the magnitudes of the energy gap valueof the host material and the energy gap value of the hole transportingmaterial can be referred to as a rough guide with respect to energytransfer. If the energy gap value of the host material is smaller thanthe energy gap value of the hole transporting material, it is difficultto excite the hole transporting material by transfer of energy from thehost material. For this reason, it is preferred that the holetransporting material have an energy gap value larger than that of thehost material in order to avoid transfer of energy from the hostmaterial to the hole transporting material.

[0064]FIG. 3 is a relating energy diagram. The materials may be selectedso that the energy gap value A of the hole transporting material islarger than the energy gap value B of the host material, as shown inFIG. 3.

[0065] A method of selecting, as a condition for prevention of energytransfer between the host and hole transporting materials, a combinationof materials such that there is no overlap between the light emissionspectrum of the host material and the absorption spectrum of the holetransporting material may be used. When this method is used, it ispreferred that the absorption spectrum of the hole transporting materialis placed on the shorter-wavelength side of the light emission spectrumof the electron transporting material.

[0066]FIGS. 4A and 4B illustrate this condition. The positionalrelationship between the spectrums in a case where transfer of energyoccurs between the host material and the hole transporting material isindicated in FIG. 4A, and the positional relationship between thespectrums in a case where transfer of energy does not occur between thehost material and the hole transporting material is indicated in FIG.4B. According to the present invention, the positional relationship ofFIG. 4B is preferred.

[0067] It is important to consider a condition other than thosedescribed above if a host material is selected such that both the energylevels of the HOMO and LUMO of the dopant material are placed in theenergy gap between the HOMO and LUMO of the host material, because insuch a case the direct-recombination light emission mechanism (secondlight emission condition) is taken into consideration.

[0068] In such a case, it is suitable to set the value of the ionizationpotential indicating the HOMO of the hole transporting material to alarger value in order to facilitate injection of the hole carrier fromthe hole transporting material to the dopant material. That is, acombination of materials is selected such that the ionization potentialof the hole transporting material is higher than that of the dopantmaterial. If the ionization potential of the hole transporting materialis excessively high, the facility with which holes are injected from theanode into the hole transporting material is reduced. In such a case, ahole injection layer may be provided between the anode and the holetransporting layer to facilitate injection.

[0069] It is thought that the dopant traps the electron carrier throughthe electron-transporting host. In a case where electrons not trapped bythe dopant reach the interface on the hole transporting layer by movingthrough the electron transporting layer, the electrons reaching theinterface enter the hole transporting layer if the difference betweenthe LUMO level of the hole transporting material and the LUMO level ofthe host material is small. In such a case, electrons are not confinedin the electron transporting layer and efficient recombination cannot beachieved. To avoid such a situation, it is desirable to set thedifference between the LUMO levels of the hole transporting material andthe electron transporting material which is a host material to a valuelarge enough to block electrons. Preferably, this difference is 0.2 eVor greater.

[0070] More concrete examples of a method of fabricating the tripletlight emitting device of the prevent invention and materials used in thedevice will next be described.

[0071] A device fabrication method of the present invention shown inFIG. 2 is performed as described below. First, a hole transportingmaterial is deposited on a substrate with an anode (ITO). Next, anelectron transporting material (host material) and a triplet lightemitting material (dopant) are codeposited. Finally, a cathode is formedby deposition. The dopant concentration at the time of codeposition ofthe host material and the dopant material is adjusted to about 8 wt %.Finally, sealing is performed to complete the organic light emittingdevice.

[0072] Materials which can be suitably used as a hole injectionmaterial, a hole transporting material, an electron transportingmaterial (host material), and a triplet light emitting material (dopantmaterial) in the device of the present invention are shown below.However, materials usable in the device of the present invention are notlimited to those shown below.

[0073] As the effective hole injecting material among organic compounds,there is a porphyrin-based compound, phthalocyanine (hereinafterreferred to as “H₂Pc”), copper phthalocyanine (hereinafter referred toas “CuPc”), or the like. In addition, a material which has a smallerionization potential than the hole transporting material to be used anda hole transporting function can also be used as the hole injectingmaterial. There is also used a material obtained by chemically doping aconductive polymer compound, for example, polyaniline or polyethylenedioxythiophene (hereinafter referred to as “PEDOT”) doped with sodiumpolystyrene sulfonate (hereinafter referred to as “PSS”). Alternatively,a polymer compound as an insulator is effective in flattening the anode,so that polyimide (hereinafter referred to as “PI”) is widely used.Furthermore, there is also used such an inorganic compound as a metalthin film of gold, platinum, or the like or a microthin film of aluminumoxide (hereinafter referred to as “alumina”).

[0074] As the effective hole transporting material, there is a materialhaving an energy gap value larger than that of the electron transportingmaterial to be used as the host material. Also, it is preferable thatthe material has a larger ionization potential than the light emittingmaterial or the absolute value of LUMO thereof is smaller than that ofthe electron transporting material by 0.2 eV or more.

[0075] Examples of the hole transporting material having a large energygap value which is preferable for the device of the present inventioninclude: 4,4′,4″-tris(N-carbazole)triphenylamine (hereinafter referredto as “TCTA”) represented by the following structural formula 1;1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene (hereinafter referredto as “o-MTDAB”) represented by the following structural formula 2;1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene (hereinafter referredto as “m-MTDAB”) represented by the following structural formula 3;1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene (hereinafter referredto as “p-MTDAB”) represented by the following structural formula 4; and4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane (hereinafterreferred to as “BPPM”) represented by the following structural formula5.

[0076] On the other hand,4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (hereinafterreferred to as “TPD”) which is an aromatic amine-based compound and usedmost widely and α-NPD as its derivative have smaller energy gap valuesthan the compounds represented by the structural formulae 1 to 5, andare therefore difficult to use for the device of the present invention.Table 1 shows the energy gap values (actually measured values) of thecompounds represented by the structural formulae 1 to 5, α-NPD, and TPD.TABLE 1 Material Energy gap [eV] TCTA 3.3 o-MTDAB 3.6 m-MTDAB 3.5p-MTDAB 3.6 BPPM 3.6 TPD 3.1 α-NPD 3.1

[0077] A stable material is preferred as an electron transportingmaterial used as a host. For example, a selection may be made from anumber of metal complexes of high stability. Materials used as a hostmaterial must have an energy gap value larger than that of the tripletlight emitting material, which is a dopant. Different host materials areselected according to the light emitting materials used. Examples ofelectron transporting materials usable as a host are shown below.

[0078] According to the present invention, as an example of a materialthat can be used as the host material with respect to a blue lightemitting material, there is a material in which light emission spectrumcan be seen at an extremely short wavelength as of ultraviolet region,for example,2,2′,2″-(1,3,5-benzenetrile)tris-[1-phenyl-1H-benzimidazole](hereinafter referred to as “TPBI”) represented by the followingstructural formula 6.

[0079] According to the present invention, examples of the host materialwith respect to the green light emitting material include:lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron (hereinafterreferred to as “LiB(PBO)₄”) represented by the following structuralformula 7;bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum(hereinafter referred to as “SAlo”) represented by the followingstructural formula 8;bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum(hereinafter referred to as “SAlt”) represented by the followingstructural formula 9; 2-(2-hydroxyphenyl)benzooxazolatolithium(hereinafter referred to as “Li(PBO)”) represented by the followingstructural formula 10; and(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron (hereinafter referredto as “B(PBO)Ph₂”) represented by the following structural formula 11.In addition to these, it is possible to use as the host material thematerial that can emit blue light.

[0080] According to the present invention, examples of the host materialwith respect to the red light emitting material include: Alq representedby the following structural formula 12;bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum (hereinafterreferred to as “SAlq”) represented by the following structural formula13; bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(hereinafter referred to as “BAlq”) represented by the followingstructural formula 14; lithiumtetra(2-methyl-8-hydroxy-quinolinato)boron(hereinafter referred to as “LiB(mq)₄”) represented by the followingstructural formula 15; (2-methyl-8-quinolinolato)-diphenylboron(hereinafter referred to as “BmqPh”) represented by the followingstructural formula 16; andbis(2-methyl-8-quinolinolato)aluminiumhydroxide (hereinafter referred toas “Almq₂(OH)”) represented by the following structural formula 17. Inaddition to these, it is possible to use as the host material thematerial that can emit blue light or the material that can emit greenlight.

[0081] Note that the energy gap values (actually measured values) inaccordance with some of the host materials described above are shown inTable 2. TABLE 2 Material Energy gap [eV] TPBI 3.5 LiB(PBO)₄ 3.1 SAlo3.2 SAlt 3.0 Alq 2.7 SAlq 3.0 LiB(mq)₄ 3.0

[0082] Examples of the triplet light emitting material as a dopantmostly include complexes having a central metal of iridium or platinum.However, any material may be adopted as long as it emits phosphorescenceat a room-temperature. As such a material, for example, there are PtOEP,Ir(ppy)₃, bis(2-phenylpyridinato-N,C^(2′))acetylacetonatoiridium(hereinafter referred to as “acacIr(ppy)₂”),bis(2-(4′-trile)-pyridinato-N,C^(2′))acetylacetonatoiridium (hereinafterreferred to as “acacIr(tpy)₂”), andbis(2-(2′-benzothienyl)pyridinato-N,C^(3′))acetylacetonatoiridium(hereinafter referred to as “acacIr(btp)₂”).

[0083] Note that the energy gap values (actually measured values) inaccordance with the dopant materials described above are shown in Table3. TABLE 3 Material Energy gap [eV] Ir(ppy)₃ 2.4 acacIr(ppy)₂ 2.4acacIr(tpy)₂ 2.4 acacIr(btp)₂ 2.3

[0084] As the electron injecting material, the electron transportingmaterial described above can be used. However, such an electrontransporting material (BCP, OXD7, or the like) that is used as the holeblocking material is low in stability, and thus it is inappropriate asthe electron injecting material. In addition, there is often used amicrothin film made of an insulator, for example, alkali metal halidesuch as lithium fluoride or alkali metal oxide such as lithium oxide.Also, an alkali metal complex such as lithium acetylacetonate(hereinafter referred to as “Li(acac)”) or 8-quinolinolato-lithium(hereinafter referred to as “Liq”) is effective.

[0085] A combination of materials is selected from the above-describedmaterials having the desired functions to be used in the organic lightemitting device of the present invention. Thus, a high-efficiencyorganic light emitting device which can be fabricated by a simplerprocess in comparison with the conventional triplet light emittingdevices, which has improved stability, and which has a light emissionefficiency substantially equal to that of the conventional triplet lightemitting devices can be provided.

[0086] Embodiments of the organic light emitting device of the presentinvention shown in FIG. 2 will be described below.

[0087] [Embodiment 1]

[0088] First, a 40 nm-thick layer of BPPM, which is a hole transportingmaterial, is deposited on glass substrate 101 with ITO film formed asanode 102 and having a thickness of about 100 nm. Hole transportinglayer 103 is thereby formed.

[0089] After fabrication of the hole transporting layer, acacIr(tpy)₂,which is a triplet light emitting material, and TPBI, which is anelectron transporting material (host material), are codeposited inproportions of about 2:23 (weight ratio). That is, acacIr(tpy)₂ isdispersed at a concentration of about 8 wt % in TPBI. A 50 nm-thickcodeposited film is thereby formed. This film is electron transportingand light emitting layer 104.

[0090] Finally, Mg and Ag are codeposited at an atomic ratio of 10:1 toform cathode 105. A 150 nm-thick film for cathode 205 is thereby formed.A triplet light emitting device which emits green light derived fromacacIr(tpy)₂ is thus obtained.

[0091]FIGS. 5A to 5D are graphs showing an initial characteristic and alight emission spectrum in this device. Even though the simple two-layerstructure was formed, a device characteristic of high efficiency, i.e.,a maximum external quantum efficiency of about 10%, was exhibited.

[0092] [Embodiment 2]

[0093] A device in accordance with the present invention was fabricatedby using a hole transporting material (satisfying the condition inaccordance with the present invention) different from that in Embodiment1.

[0094] First, a 40 nm-thick layer of o-MTDAB, which is a holetransporting material, is deposited on glass substrate 101 with ITO filmformed as anode 102 and having a thickness of about 100 nm. Holetransporting layer 103 is thereby formed.

[0095] After fabrication of the hole transporting layer, acacIr(tpy)₂,which is a triplet light emitting material, and TPBI, which is anelectron transporting material (host material), are codeposited inproportions of about 2:23 (weight ratio). That is, acacIr(tpy)₂ isdispersed at a concentration of about 8 wt % in TPBI. A 50 nm-thickcodeposited film is thereby formed. This film is electron transportingand light emitting layer 104.

[0096] Finally, Mg and Ag are codeposited at an atomic ratio of 10:1 toform cathode 205. A 150 nm-thick film for cathode 205 is thereby formed.A triplet light emitting device which emits green light derived fromacacIr(tpy)₂ is thus obtained.

[0097]FIGS. 6A to 6D are graphs showing an initial characteristic and alight emission spectrum of this device. A high-efficiency device can befabricated as in Embodiment 1.

[0098] [Embodiment 3]

[0099] An organic light emitting device in accordance with the presentinvention was fabricated by using as a host material a hole transportingmaterial (satisfying the condition in accordance with the presentinvention) different from that in Embodiment 1. The fabrication methodis the same as that in Embodiments 1 and 2. BPPM is used as a holetransporting material, SAlt is used as a host, i.e., the electrontransporting material, and acacIr(tpy)₂ is used as a dopant. A tripletlight emitting device which emits green light derived from acacIr(tpy)₂can be obtained.

[0100]FIGS. 7A to 7D show an initial characteristic and a light emissionspectrum of this device. A high-efficiency device having a lightemission efficiency substantially equal to that in the conventionaltriplet light emitting devices can be fabricated as in Embodiment 1 or2.

[0101] [Embodiment 4]

[0102] By using a triplet light emitting material different fromEmbodiment 1, 2, or 3 as a dopant, an organic light emitting devicehaving a light emission color different from that of Embodiment 1, 2, or3 is prepared. The method for preparation is the same as that ofEmbodiments 1, 2, and 3. BPPM is used as the hole transporting material,TPBI is used as the electron transporting material, andbis(2-(2′,4′-difluorophenyl)pyridinato-N,C2′)picolatoiridium is used asthe dopant. It is possible to obtain the triplet light emitting devicewhich emits blue light derived from the dopant material.

[0103]FIGS. 8A to 8D show an initial characteristic and a light emissionspectrum of this device. A high-efficiency device having a lightemission efficiency substantially equal to that in the conventionaltriplet light emitting devices can be fabricated as in Embodiment 1, 2,or 3.

Comparative Example 1

[0104] A device of a structure similar to that of the conventionaltriplet light emitting device shown in FIG. 12 was manufactured and itscharacteristics were compared with those of the devices of the presentinvention.

[0105] First, a 40 nm-thick layer of α-NPD, which is a hole transportingmaterial, is deposited on glass substrate 1101 with ITO film formed asanode 1102 and having a thickness of about 100 nm. Hole transportinglayer 1103 is thereby formed.

[0106] After fabrication of the hole transporting layer, acacIr(tpy)₂,which is a triplet light emitting material, and CBP, which is a hostmaterial, are codeposited in proportions of about 2:23 (weight ratio).That is, acacIr(tpy)₂ is dispersed at a concentration of about 8 wt % inCBP. A 50 nm-thick codeposited film is thereby formed. This film islight emitting layer 1104.

[0107] After the formation of the light emitting layer, a 20 nm-thickfilm of BCP, which is a hole blocking material, is deposited to formhole blocking layer 1105. A 30 nm-thick film of Alq, which is anelectron transporting material is deposited to form electrontransporting layer 1106.

[0108] Finally, Mg and Ag are codeposited at an atomic ratio of 10:1 toform cathode 1107. A 150 nm-thick film for cathode 1107 is therebyformed. A triplet light emitting device which emits green light derivedfrom acacIr(tpy)₂ is thus obtained.

[0109]FIGS. 9A to 9D show an initial characteristic and a light emissionspectrum of this device. From comparison between this comparativeexample and each of Embodiments 1, 2, and 3, it can be understood thatthe device of the present invention in each Embodiment has the same highefficiency as the conventional device. It was confirmed thatsufficiently high device characteristics were exhibited even though nohole blocking layer was used.

Comparative Example 2

[0110] In this comparative example, characteristics of a triplet lightemitting device of a two-layer structure in which a hole transportingmaterial not satisfying the device conditions in accordance with thepresent invention is used are examined.

[0111] The same fabrication method as that in the Embodiments of thepresent invention is used. However, a combination of a hole transportingmaterial and a host material is used such that the energy gap value ofthe hole transporting material used is smaller than that of the hostmaterial. TPD is used as the hole transporting material, TPBI is used asthe host material, which is an electron transporting material, andacacIr(tpy)₂ is used as a dopant.

[0112]FIGS. 10A to 10D show an initial characteristic and a lightemission spectrum of this device. The device using TPD as a holetransporting material has a considerably low light emission efficiencyfor a triplet light emitting device. A spectral component (about 400 nm)corresponding to emission from TPD other than emission from acacIr(tpy)₂is observed, as seen in the light emission spectrum. A reduction inefficiency results from this. Thus, the initial characteristic of thedevice is inferior if a material not satisfying the condition is used.

Comparative Example 3

[0113] In this comparative example, characteristics of a triplet lightemitting device of a two-layer structure in which a hole transportingmaterial not satisfying the device conditions in accordance with thepresent invention is used as in Comparative Example 2 are examined.

[0114] The same fabrication method as that in the Embodiments of thepresent invention is used. However, a combination of a hole transportingmaterial and a host material is used such that the energy gap value ofthe hole transporting material used is smaller than that of the hostmaterial. In this example, α-NPD is used as the hole transportingmaterial, TPBI is used as the host material, which is an electrontransporting material, and acacIr(tpy)₂ is used as a dopant.

[0115]FIGS. 11A to 11D show an initial characteristic and a lightemission spectrum of this device. The device using α-NPD as a holetransporting material has a considerably low light emission efficiencyfor a triplet light emitting device, as in Comparative Example 2. Aspectral component (about 440 nm) corresponding to emission from α-NPDwhich is a hole transporting material is observed, as in ComparativeExample 2. A reduction in efficiency results from this. Thus, theinitial characteristic of the device is inferior if a material notsatisfying the condition is used.

[0116] If the present invention is carried out, a triplet light emittingdevice having a light emission efficiency substantially equal to that ofthe conventional triplet light emitting devices can be obtained in asimple device structure. Also, the layer in which an unstable materialis used is removed to enable a stable organic light emitting device tobe provided.

What is claimed is:
 1. A light emitting device comprising an anode, anorganic compound film, and a cathode, the organic compound filmcomprising: a hole transporting layer containing a hole transportingmaterial; and an electron transporting layer in contact with the holetransporting layer and containing an electron transporting material,wherein a light emitting material capable of emitting light from atriplet excited state is added in the electron transporting layer.
 2. Alight emitting device according to claim 1, wherein an energy differencebetween a highest occupied molecular orbit level and a lowest unoccupiedmolecular orbit level in the hole transporting material is larger thanthat between a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level in the electron transporting material.3. A light emitting device according to claim 1, wherein an absorptionspectrum of the hole transporting material and a light emission spectrumof the electron transporting material do not overlap each other.
 4. Alight emitting device according to claim 1, wherein an absorptionspectrum of the hole transporting material is on a shorter-wavelengthside of a light emission spectrum of the electron transporting material.5. A light emitting device according to claim 1, wherein both a highestoccupied molecular orbit level and a lowest unoccupied molecular orbitlevel of the light emitting material are in an energy gap between ahighest occupied molecular orbit level and a lowest unoccupied molecularorbit level of the electron transporting material.
 6. A light emittingdevice according to claim 1, wherein an ionization potential of the holetransporting material is equal to or larger than that of an ionizationpotential of the light emitting material.
 7. A light emitting deviceaccording to claim 1, wherein an absolute value of a lowest unoccupiedmolecular orbit level of the hole transporting material is smaller by0.2 eV or more than that of a lowest unoccupied molecular orbit level ofthe electron transporting material.
 8. A light emitting device accordingto claim 1, wherein an ionization potential of the hole transportingmaterial is equal to or larger than that of ionization potential of thelight emitting material, and an absolute value of a lowest unoccupiedmolecular orbit level of the hole transporting material is smaller by0.2 eV or more than that of a lowest unoccupied molecular orbit level ofthe electron transporting material.
 9. A light emitting device accordingto claim 1, wherein used as the hole transporting material is oneselected from the group consisting of4,4′,4″-tris(N-carbazole)triphenylamine,4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane,1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene,1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene, and1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene.
 10. A light emittingdevice according to claim 1, wherein used as the electron transportingmaterial is one selected from the group consisting of2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzoimidazole],lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron,bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum,bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum,2-(2-hydroxyphenyl)benzooxazolatolithium,(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron,tris(8-quinolinolato)-aluminum,bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum,lithiumtetra(2-methyl-8-hydroxy-quinolinato)boron,(2-methyl-8-quinolinolato)-diphenylboron, andbis(2-methyl-8-quinolinolato)aluminiumhydroxide.
 11. A light emittingdevice comprising an anode, an organic compound film, and a cathode, theorganic compound film comprising: a hole injection layer in contact withthe anode; and a hole transporting layer containing a hole transportingmaterial; an electron transporting layer in contact with the holetransporting layer and containing an electron transporting material,wherein a light emitting material capable of emitting light from atriplet excited state is added in the electron transporting layer.
 12. Alight emitting device according to claim 11, wherein an energydifference between a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level in the hole transporting material islarger than that between a highest occupied molecular orbit level and alowest unoccupied molecular orbit level in the electron transportingmaterial.
 13. A light emitting device according to claim 11, wherein anabsorption spectrum of the hole transporting material and a lightemission spectrum of the electron transporting material do not overlapeach other.
 14. A light emitting device according to claim 11, whereinan absorption spectrum of the hole transporting material is on ashorter-wavelength side of a light emission spectrum of the electrontransporting material.
 15. A light emitting device according to claim11, wherein both a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level of the light emitting material are inan energy gap between a highest occupied molecular orbit level and alowest unoccupied molecular orbit level of the electron transportingmaterial.
 16. A light emitting device according to claim 11, wherein anionization potential of the hole transporting material is equal to orlarger than that of an ionization potential of the light emittingmaterial.
 17. A light emitting device according to claim 11, wherein anabsolute value of a lowest unoccupied molecular orbit level of the holetransporting material is smaller by 0.2 eV or more than that of a lowestunoccupied molecular orbit level of the electron transporting material.18. A light emitting device according to claim 11, wherein an ionizationpotential of the hole transporting material is equal to or larger thanthat of ionization potential of the light emitting material, and anabsolute value of a lowest unoccupied molecular orbit level of the holetransporting material is smaller by 0.2 eV or more than that of a lowestunoccupied molecular orbit level of the electron transporting material.19. A light emitting device according to claim 11, wherein used as thehole transporting material is one selected from the group consisting of4,4′,4″-tris(N-carbazole)triphenylamine,4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane,1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene,1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene, and1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene.
 20. A light emittingdevice according to claim 11, wherein used as the electron transportingmaterial is one selected from the group consisting of2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzoimidazole],lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron,bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum,bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum,2-(2-hydroxyphenyl)benzooxazolatolithium,(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron,tris(8-quinolinolato)-aluminum,bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum,lithiumtetra(2-methyl-8-hydroxy-quinolinato)boron,(2-methyl-8-quinolinolato)-diphenylboron, andbis(2-methyl-8-quinolinolato)aluminiumhydroxide.
 21. A light emittingdevice comprising an anode, an organic compound film, and a cathode, theorganic compound film comprising: a hole transporting layer containing ahole transporting material; an electron transporting layer in contactwith the hole transporting layer and containing an electron transportingmaterial; and an electron injection layer in contact with the cathode;wherein a light emitting material capable of emitting light from atriplet excited state is added in the electron transporting layer.
 22. Alight emitting device according to claim 21, wherein an energydifference between a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level in the hole transporting material islarger than that between a highest occupied molecular orbit level and alowest unoccupied molecular orbit level in the electron transportingmaterial.
 23. A light emitting device according to claim 21, wherein anabsorption spectrum of the hole transporting material and a lightemission spectrum of the electron transporting material do not overlapeach other.
 24. A light emitting device according to claim 21, whereinan absorption spectrum of the hole transporting material is on ashorter-wavelength side of a light mission spectrum of the electrontransporting material.
 25. A light emitting device according to claim21, wherein both a highest occupied molecular orbit level and a lowestunoccupied molecular orbit level of the light emitting material are inan energy gap between a highest occupied molecular orbit level and alowest unoccupied molecular orbit level of the electron transportingmaterial.
 26. A light emitting device according to claim 21, wherein anionization potential of the hole transporting material is equal to orlarger than that of an ionization potential of the light emittingmaterial.
 27. A light emitting device according to claim 21, wherein anabsolute value of a lowest unoccupied molecular orbit level of the holetransporting material is smaller by 0.2 eV or more than that of a lowestunoccupied molecular orbit level of the electron transporting material.28. A light emitting device according to claim 21, wherein an ionizationpotential of the hole transporting material is equal to or larger thanthat of ionization potential of the light emitting material, and anabsolute value of a lowest unoccupied molecular orbit level of the holetransporting material is smaller by 0.2 eV or more than that of a lowestunoccupied molecular orbit level of the electron transporting material.29. A light emitting device according to claim 21, wherein used as thehole transporting material is one selected from the group consisting of4,4′,4″-tris(N-carbazole)triphenylamine,4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane,1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene,1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene, and1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene.
 30. A light emittingdevice according to claim 21, wherein used as the electron transportingmaterial is one selected from the group consisting of2,2′,2′-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzoimidazole],lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron,bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum,bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum,2-(2-hydroxyphenyl)benzooxazolatolithium,(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron,tris(8-quinolinolato)-aluminum,bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum,lithiumtetra(2-methyl-8-hydroxy-quinolinato)boron,(2-methyl-8-quinolinolato)-diphenylboron, andbis(2-methyl-8-quinolinolato)aluminiumhydroxide.
 31. A light emittingdevice comprising an anode, an organic compound film, and a cathode, theorganic compound film comprising: a hole injection layer in contact withthe anode; a hole transporting layer containing a hole transportingmaterial; an electron transporting layer in contact with the holetransporting layer and containing an electron transporting material; andan electron injection layer in contact with the cathode, wherein a lightemitting material capable of emitting light from a triplet excited stateis added in the electron transporting layer.
 32. A light emitting deviceaccording to claim 31, wherein an energy difference between a highestoccupied molecular orbit level and a lowest unoccupied molecular orbitlevel in the hole transporting material is larger than that between ahighest occupied molecular orbit level and a lowest unoccupied molecularorbit level in the electron transporting material.
 33. A light emittingdevice according to claim 31, wherein an absorption spectrum of the holetransporting material and a light emission spectrum of the electrontransporting material do not overlap each other.
 34. A light emittingdevice according to claim 31, wherein an absorption spectrum of the holetransporting material is on a shorter-wavelength side of a lightemission spectrum of the electron transporting material.
 35. A lightemitting device according to claim 31, wherein both a highest occupiedmolecular orbit level and a lowest unoccupied molecular orbit level ofthe light emitting material are in an energy gap between a highestoccupied molecular orbit level and a lowest unoccupied molecular orbitlevel of the electron transporting material.
 36. A light emitting deviceaccording to claim 31, wherein an ionization potential of the holetransporting material is equal to or larger than that of an ionizationpotential of the light emitting material.
 37. A light emitting deviceaccording to claim 31, wherein an absolute value of a lowest unoccupiedmolecular orbit level of the hole transporting material is smaller by0.2 eV or more than that of a lowest unoccupied molecular orbit level ofthe electron transporting material.
 38. A light emitting deviceaccording to claim 31, wherein an ionization potential of the holetransporting material is equal to or larger than that of ionizationpotential of the light emitting material, and an absolute value of alowest unoccupied molecular orbit level of the hole transportingmaterial is smaller by 0.2 eV or more than that of a lowest unoccupiedmolecular orbit level of the electron transporting material.
 39. A lightemitting device according to claim 31, wherein used as the holetransporting material is one selected from the group consisting of4,4′,4″-tris(N-carbazole)triphenylamine,4,4′-bis[N,N-bis(3-methylphenyl)-amino]-diphenylmethane,1,3,5-tris[N,N-bis(2-methylphenyl)-amino]-benzene,1,3,5-tris[N,N-bis(3-methylphenyl)-amino]-benzene, and1,3,5-tris[N,N-bis(4-methylphenyl)-amino]-benzene.
 40. A light emittingdevice according to claim 31, wherein used as the electron transportingmaterial is one selected from the group consisting of2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzoimidazole],lithiumtetra(2-(2-hydroxyphenyl)benzooxazolatoboron,bis(2-(2-hydroxyphenyl)benzooxazolato)(triphenylsilanolato)aluminum,bis(2-(2-hydroxyphenyl)benzothiazolato)(triphenylsilanolato)aluminum,2-(2-hydroxyphenyl)benzooxazolatolithium,(2-(2-hydroxyphenyl)benzooxazolato)-diphenylboron,tris(8-quinolinolato)-aluminum,bis(2-methyl-8-quinolinolato)(triphenylsilanolato)aluminum,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum,lithiumtetra(2-methyl-8-hydroxy-quinolinato)boron,(2-methyl-8-quinolinolato)-diphenylboron, andbis(2-methyl-8-quinolinolato)aluminiumhydroxide.